Electrophotographic printing is well known and has been widely refined. For example, today, almost every office and indeed some homes have electrophotographic copiers. The industry has grown to the point where it is now a highly competitive multi-billion dollar industry. In most instances, these home and office copiers are capable of providing only about a few copies per minute.
In electrophotography, images are photoelectrically formed on a photoconductive layer mounted on a conductive base. Liquid or dry developer or toner mixtures may be used to develop the requisite image.
Liquid toner dispersions for use in the process are formed by dispersing dyes or pigments and natural or synthetic resin materials in a low dielectric constant carrier liquid. Charge control agents are added to the liquid toner dispersions to aid in charging the pigment and dye particles to the requisite polarity for proper image formation on the desired substrate.
The photoconductive layer is sensitized by electrical charging whereby electrical charges are uniformly distributed over the surface. The photoconductive layer is then exposed by projecting or alternatively by writing over the surface with a laser, L.E.D., or the like. The electrical charges on the photoconductive layer are conducted away from the areas exposed to light with an electrostatic charge remaining in the image area. The charged pigment and/or dye particles from the liquid toner dispersion contact and adhere to the image areas of the photoconductive layer. The image is then transferred to the desired substrate, such as a travelling web of paper or the like.
In contrast to office and home copiers, high speed electrophotographic printing presses are being developed wherein successive images are rapidly formed on the photoconductive medium for rapid transfer to carrier sheets or the like travelling at speeds of greater than 100 ft./min. and even at speeds of from 300-500 ft./min. These high speed presses are capable of delivering 10 million copies per month with web or copy widths being on the order of 20 inches or greater.
In a high speed printing press, it is necessary that the image, after application of toner, be thoroughly fixed and dried prior to later (downstream) operations such as punching, perforating, rewinding, folding and/or sheeting in order that the final printed production is of requisite quality and press parts remain clean and free of toner which can mark the web. Furthermore, to dry and fuse a variety of different color toners efficiently, it is highly desirable to heat the travelling web, therefore volatilizing the dielectric carrier liquid and fusing the pigment and/or dye particles and associated synthetic resin binder to the web in such manner that image smearing is inhibited by minimizing contact of the travelling web surfaces with conveyor rollers and the like.
Further, due to the heat requirements of the drying process, and the attendant energy costs associated with same, it is highly desirable to provide a dryer-fuser apparatus that may successfully operate at high speed, while minimizing the energy input requirements for the heating process.
Due to the flammable nature of the dielectric carrier liquid utilized in such electrophotographic processes, it is highly desirable to provide a drying apparatus which maintains the drying atmosphere at a level that is substantially less than the lower flammability level of the carrier liquid. That is, at certain levels, the volatilized dielectric carrier liquid can provide a source of considerable danger in that in the presence of an external flame or spark the volatiles may ignite. For this reason, it is desirable to provide a dryer-fuser apparatus whereby fresh make-up air can be readily admitted to the drying zone so as to aid in maintaining the carrier liquid/atmospheric gas content well below that of the lower flammability level of the carrier liquid.
Further, it is desirable to provide a system to monitor the concentration of volatile carrier liquid in the dryer to control the rate at which the volatile gas containing atmosphere is exhausted from the apparatus in response thereto. At increasingly higher volatile levels, it is desirable to halt the travel of the web through the dryer altogether.